Presently, it is not uncommon for vehicles, such as trucks and buses, to be equipped with an exhaust brake. Fundamentally, an exhaust brake need only comprise some means for restricting the flow of exhaust gas from an internal combustion engine. Restricting the exhaust gas increases the exhaust manifold pressure, i.e. "back pressure". The exhaust manifold pressure may be used to oppose the motion of the engine pistons, converting the kinetic energy of the pistons into thermal energy. The engine and vehicle may be slowed by dissipating the thermal energy that is generated. Selective restriction of the flow of exhaust gas from the engine may therefore be used to selectively brake or not brake a vehicle.
An exhaust brake may be used to complement and/or enhance the braking achieved with compression release braking systems, and more specifically compression release systems that use exhaust gas recirculation (EGR). EGR may be used to increase the braking power of a compression release braking system. EGR returns exhaust gas to a cylinder from the exhaust manifold to boost the mass of gas in the cylinder for each compression release event. In order to carry out EGR in a compression release braking system, high exhaust gas back pressure (i.e. exhaust manifold pressure) is required to charge the cylinders with exhaust gas during the intake cycle. The increase in compression release braking realized from EGR is therefore related to the amount of exhaust gas that is returned to the cylinder, which in turn is related to the exhaust manifold pressure. Thus, the increased exhaust manifold pressure produced by an exhaust brake may be particularly useful for use in compression release braking systems that employ EGR.
Exhaust manifold pressure produced by an exhaust brake may also be useful in warming an engine during positive power operation. A cold engine may be more quickly warmed by placing the engine under load during positive power operation. Partially closing an exhaust brake during positive power creates an engine load because it makes it more difficult for the pistons to cycle in the cylinders. The exhaust brake creates this load by backing up warm exhaust gases in the engine and exhaust manifold, which also helps to warm the engine at an accelerated pace.
An exhaust brake may generate exhaust back pressure in the exhaust manifold. The back pressure in the exhaust manifold may be proportional to the speed of the engine. The faster the engine runs, the more frequently exhaust gas is discharged to the exhaust manifold, and consequently the higher the exhaust manifold pressure. The increased manifold pressure produced by an exhaust brake translates into decreased removal of heat from the engine. Engines cannot withstand unlimited amounts of exhaust manifold pressure or the accompanying heat. Accordingly, exhaust brakes have been designed so that the thermal limits of an engine are not exceeded when the engine is running at maximum speed. This type of exhaust brake design optimizes engine and exhaust braking only for one engine condition; maximum speed. Very frequently, however, engine braking and exhaust braking is carried out at less than maximum engine speed. As engine speed decreases, so does exhaust manifold pressure, and as a result the level of braking realized is decreased.
Some exhaust brakes have been designed to provide a fixed maximum level of back pressure over a range of engine speeds. In such exhaust brakes, control of the exhaust manifold pressure may be achieved by control of the restriction of exhaust gas flow by the exhaust brake. These exhaust brakes typically allow back pressure to build to a preset limit. Back pressure which exceeds the preset limit is relieved via a bypass around the closed exhaust brake. For example, U.S. Pat. No. 5,638,926 to McCrickard discloses an exhaust brake having a main tube and a bypass tube. During exhaust braking, the main tube is blocked with a rotatable valve. Back pressure is relieved by opening a bypass valve located at the far end of the bypass tube. Also see U.S. Pat. Nos. 4,750,459 and 4,682,674 to Schmidt, and U.S. Pat. No. 5,372,109 which disclose alternative bypass arrangements for an exhaust brake.
One restriction of other exhaust brakes is that they may be limited to providing one level of exhaust braking. For example, an exhaust brake, such as the one disclosed in the above-referenced patent to McCrickard, does not provide a means for varying the pressure level which results in opening the bypass valve. The bypass valve must be set to open at a back pressure which will not cause the engine temperature to exceed a critical level when the engine is running at maximum speed. As it turns out, however, this preset back pressure is less than the maximum back pressure which could be used at lower engine speeds. This preset is therefore only optimal for maximum engine speed. Accordingly, there is a need for exhaust brakes which provide variable levels of exhaust back pressure.
The present invention improves exhaust brake performance by selective variation of the back pressure at which a bypass valve opens. Variation of this pressure in response to one or more engine conditions can optimize the back pressure for a range of engine speeds. One way of determining the optimal back pressure for a given engine speed is to sense the temperature of the engine. The critical temperature of the engine may be tracked for a range of engine speeds by varying the bypass pressure so that engine temperature is a constant safe margin below critical temperature for each engine speed. Tracking the critical temperature in this way may result in selection of a moderate back pressure at the maximum engine speed, and a gradually increasing back pressure down through the speed range to the lowest engine speed. Variable back pressure through bypass control may enable tailoring the exhaust braking effect to optimize braking in response to the variation of conditions such as engine speed, exhaust pressure, engine temperature, EGR activation, and/or compression release braking activation. Control of the bypass pressure may be particularly beneficial in combination braking systems which may require high back pressure at low engine speeds and low back pressure at high engine speeds.
Bypass systems should also be constructed to remain operable under the harsh conditions of an exhaust brake. Exhaust gas typically contains carbon particles, water moisture, and other contaminants within it. Exposure of the moving parts of a bypass system to exhaust gas and its contaminants can cause the moving parts to rust and become gummed up with carbon. Bypass valves, such as the one disclosed in the above-referenced Schmidt patents, have been known to become inoperable because of the build up of contaminants on the moving parts in the system. Accordingly, there is also a need for an exhaust brake bypass system that is less prone to malfunction as a result of carbon, rust, or other contaminant build up on the moving parts of the bypass.
Furthermore, bypass systems should preferably be designed to avoid the exposure of heat sensitive elements of the bypass from being over exposed to high temperature exhaust gas. A bypass system may use a spring and/or electronic activators to open and close the bypass. These types of elements may not operate well under fluctuating or extreme temperature conditions. Accordingly, there is a need for an exhaust brake with a bypass activator that has an acceptable exposure to high temperature exhaust gas.
One of the designs described herein is a bolt-on bypass circuit which may be very effective at reducing the exposure of the bypass spring and/or electronic activators to exhaust gas temperatures. A bolt-on bypass may also add the benefit of manufacturability which allows for a fixed flow area device or a variable area device with minimal manufacturing set up changes. A bolt-on bypass may be used with an exhaust brake that is pre-configured to accept the bypass. The exhaust brake may be provided originally with two or more plugged openings. The openings may be unplugged when a bolt-on bypass is added to provide exhaust gas flow to and from the bypass.